Observations of Surface Textures That Are Indicative of Lava Sheet Inflation In Monogenetic Flow Fields: Insights from the McCartys and Carrizozo Flow Fields, NM

The importance of sheet flow inflation processes in the development of basaltic terrains has gained both attention and acceptance within the last 15 years. Inflated sheets are known to have occurred within the distal margins of large shield volcanoes, flood-lava deposits, and submarine lava flows. Inflation features (inflation pits, tumuli, and inflated flow margins) have been observed within the Carrizozo and McCartys flow fields, suggesting that sheet inflation is also an emplacement style associated with monogenetic vents. In agreement with past works, we observed and reported on the same types of inflation features on both flows. Here, we present our field observations of flow textures associated with these two vents and compare them with published observations of inflated sheet flow textures. The McCartys and Carrizozo flow fields represent monogenetic, basaltic eruptions of lavas over < 1 degree slopes, at ~3000 and 5000 years ago respectively along the Rio Grande Rift System in New Mexico. We observed the following surface textures that together are suggestive of inflation, including: 1) grooved/lineated, meters-wide hummocks/swales, 2) smooth pahoehoe toes and ropes, 3) concentrated areas of broken slabs of smooth crust (sometimes reverse imbricated), 4) lava coils, 5) lava balls, 6) squeeze-ups, and 7) flow scars that form “wakes” associated with obstacles. In cross section the surface textures only occur within a thin upper layer over a continuous massive interior. Surface textures can abruptly change from smooth to broken and rough with no disruption of the interior beneath the surface transition. The surface layer is typically segregated from the massive interior by a vesicle rich zone. We conclude that inflated lava sheets are an emplacement-related morphology that is pervasive throughout basaltic terrains, and is indicative of a constant local lava supply over low slopes, regardless of the volcano-tectonic setting or vent conditions.